It takes a lot of force to pull it apart far enough for the front and rear pieces to slide together.

It helps to give the seat a bath in very hot water (wear protective gloves when handling) and to use a rubber lubricant like Ru-Glide, but dish washing soap will work, too.

In the past I have used a come-a-long and BIG turnbuckles (on separate ocassions) to provide the stretch. You will need two stout anchor points (I have stretched seats between two support columns and between the trailer hitches of two cars parked back-to-back). Whatever device you use, it will be much easier if you have the ability to make fine adjustments. It was easier with the turnbuckles for that reason.

You will also need to make a bracket to attach the cable to the two bolt holes on the rear seat bracket. If you use rope instead of wire rope or chain, use a very heavy one or it will stretch more than the seat.

With enough mechanical advantage, it is a (relatively) easy task. The second one you do will be a lot easier than the first .